One out of 9 women in the USA is likely to develop one of a variety of hormone-dependent (HDT) or-independent (HIT) breast cancer during her lifetime. The etiology of these breast cancers are undefined but studies indicate an important role of hormones in normal breast cell proliferation, as well as in cancer development. Two hormone-related functions also appear to significantly decrease the risk for breast cancer: these are decreased ovarian activity and full term pregnancy soon after puberty. Pregnancy and lactation is followed by involution of alveolar cells. Our overall goal is to be able to understand the mechanisms by which hormone-related proliferation and involution in mammary epithelial cells contribute to the development or prevention of different studies. The aim of our current studies is to conduct molecular analysis of known and novel oncogenes in MNU-induced precancerous and cancerous lesions induced in different hormonal, growth factor and other mitogenic environments. We intend to determine the factors regulating the origin of HDT and HIT. We will use endocrine manipulation to induce parity-like refractiveness to mammary carcinogenesis in mammary epithelial cells of virgin rats without their having to go through pregnancy. These studies should provide a more comprehensive model for human breast cancer studies, as well as expand basic knowledge of the mechanism(s) involved in transformation of normal cells to preneoplastic and neoplastic lesions. During the past years, we have adapted many cell and molecular biological technologies and have succeeded in using a recently developed expression cloning system to clone a novel oncogene from MNU-induced mammary lesions from mice and rats. These multifaceted resources, in addition to our vast experience with in vivo and in vitro transformation and cell culture technics will be utilized to begin to answer some major questions pertinent to human breast cancer. The specific aims for the proposed project are: 1) Molecular analysis of preneoplastic lesions and tumors induced by MNU in vitro and in vivo in mammary epithelial cells proliferating under the influence of various mitogens; 2) Determine whether or not growth factors are supportive of mammary carcinogenesis by MNU; 3) Determination of factors regulating the development of HDT and HIT varieties of mammary tumors in rodents; 4) Determine the relationship between parity and refractoriness to mammary tumor development: induction of parity-like refractoriness in virgin female rats.